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Quantum Scarring in a Two-Dimensional Elliptical Harmonic Oscillator

Aleksi Ruhanen

Published Jan 1, 2019• 0 citations
OpenAlex

Abstract

Perturbation-induced quantum scarring is a recently discovered phenomenonin a quantum well perturbed by local potential bumps.In this phenomenon,the probability density of an eigenstate in a perturbed system is enhanced along a periodic orbit of the unperturbed classical counterpart. In this Thesis, we examine perturbation-induced scarring in a two-dimensional anisotropic (elliptic) oscillator characterized by the anisotropy parameter, i.e., the frequency ratio of the confining potential. In particular, we show that some eigenstates of the perturbed oscillator are scarred by the so-called Lissajous orbits occurring at specific anisotropy parameters. Furthermore, the stability of these Lissajous scars is preliminarily analyzed.

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A quantum scar-an enhancement of a quantum probability density in the vicinity of a classical periodic orbit-is a fundamental phenomenon connecting quantum and classical mechanics. Here we demonstrate that some of the eigenstates of the perturbed two-dimensional anisotropic (elliptic) harmonic oscillator are strongly scarred by the Lissajous orbits of the unperturbed classical counterpart. In particular, we show that the occurrence and geometry of these quantum Lissajous scars are connected to the anisotropy of the harmonic confinement, but unlike the classical Lissajous orbits the scars survive under a small perturbation of the potential. This Lissajous scarring is caused by the combined effect of the quantum (near) degeneracies in the unperturbed system and the localized character of the perturbation. Furthermore, we discuss experimental schemes to observe this perturbation-induced scarring.

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•Nov 6, 2006•5 citations
Perturbation-induced quantum scarring is a recently discovered phenomenonin a quantum well perturbed by local potential bumps.In this phenomenon,the probability density of an eigenstate in a perturbed system is enhanced along a periodic orbit of the unperturbed classical counterpart. In this Thesis, we examine perturbation-induced scarring in a two-dimensional anisotropic (elliptic) oscillator characterized by the anisotropy parameter, i.e., the frequency ratio of the confining potential. In particular, we show that some eigenstates of the perturbed oscillator are scarred by the so-called Lissajous orbits occurring at specific anisotropy parameters. Furthermore, the stability of these Lissajous scars is preliminarily analyzed.
A quantum scar-an enhancement of a quantum probability density in the vicinity of a classical periodic orbit-is a fundamental phenomenon connecting quantum and classical mechanics. Here we demonstrate that some of the eigenstates of the perturbed two-dimensional anisotropic (elliptic) harmonic oscillator are strongly scarred by the Lissajous orbits of the unperturbed classical counterpart. In particular, we show that the occurrence and geometry of these quantum Lissajous scars are connected to the anisotropy of the harmonic confinement, but unlike the classical Lissajous orbits the scars survive under a small perturbation of the potential. This Lissajous scarring is caused by the combined effect of the quantum (near) degeneracies in the unperturbed system and the localized character of the perturbation. Furthermore, we discuss experimental schemes to observe this perturbation-induced scarring.
Quantum scars are tracks of enhanced probability densities along unstable classical periodic orbits in quantum chaotic systems. They are a visual representation of classical-quantum correspondence as well as an example of quantum suppression of chaos. A new type of quantum scarring was recently discovered in radially symmetric two-dimensional potential wells perturbed by localized impurities. These perturbation-induced scars show potential to be utilized in controlled applications in classically chaotic quantum systems due to the ability to efficiently propagate wave packets along the scars. In this thesis, the focus is on special linearly shaped perturbation-induced quantum scars which correspond to the classical motion of a ball bouncing between two walls. These bouncing ball quantum scars have potential to be used as efficient channels in quantum transport, but their behavior is still mostly unexplored. This thesis studies how bouncing ball scars could be controlled by changing the perturbation parameters and how stable these scars are against external noise. Both of these features are relevant when utilization in experimental applications is considered. Bouncing ball scars are studied in a radially symmetric two-dimensional (2D) potential well perturbed by a singular Gaussian-like potential bump or a dip, which are placed into the same location. For the stability calculations, randomly placed smaller Gaussian bumps are also placed into the potential with a constant uniform mean density in addition to the main perturbation. Imaginary time propagation algorithm is then applied to solve the 2D Schrödinger equation in our model systems. Bouncing ball scars are detected from the solved eigenstates by integrating the probability densities of the states inside narrow detection boxes. The results indicate that the singular potential bump and the dip affect the same states of the clean system in different ways. The bump seems to pin the bouncing ball scars onto itself, when the dip causes the scars to avoid it. This means that the different perturbation types have the opposite scar orientation preferences. The abundance of the bouncing ball scars seems to also be connected to the perturbation parameters, and the results indicate that the parameters could be optimized in order to maximize the abundance. The stability results show impressive robustness against external noise. The number of bouncing ball scars remain nearly constant with weaker noise and even when the noise bumps were 10% from the main perturbation size, a notable amount of bouncing ball scars persist. It also seems that the scars retain their orientations even in the noisy systems.
Chaos plays a crucial role in numerous natural phenomena, but its quantum nature has remained large elusive. One intriguing quantum-chaotic phenomenon is the scarring of a single-particle wavefunction, where the quantum probability density is enhanced in the vicinity of a classical periodic orbit. These quantum scars illustrate the quantum suppression of classical chaos, offering a unique way to explore the classical-quantum relationship beyond conventional limits. In this study, we establish an ergodicity theorem for slacking a group of adjacent eigenstates, revealing the aspect of antiscarring -- the reduction of probability density along a periodic orbit generating the corresponding scars. We thereafter apply these two concepts to variational scars in a disordered quantum well, and finally discuss their broader implications, suggesting potential experimental approaches to observe this phenomenon.
The semiclassical treatment of the two-dimensional harmonic oscillator provides an instructive example of the relation between classical motion and the quantum mechanical energy spectrum. We extend previous work on the anisotropic oscillator with incommensurate frequencies and the isotropic oscillator to the case with commensurate frequencies for which the Lissajous curves appear as classical periodic orbits. Because of the three different scenarios depending on the ratio of its frequencies, the two-dimensional harmonic oscillator offers a unique way to explicitly analyze the role of symmetries in classical and quantum mechanics.
Quantum scar is an intriguing phenomenon in quantum or wave dynamics that the wavefunction takes an exceptionally large value around an unstable periodic orbit. It has attracted much attention and advances the understanding of the semiclassical quantization. Most of previous researches involving quantum scars focus on hard-wall quantum billiards. Here we investigate the quantum billiard with a smooth confinement potential which possesses complex classical dynamics. We demonstrate that the semiclassical quantization approach works well for both the stable and unstable classical periodic orbit, besides the fact that the shape of the orbits varies as the energy increases or even the stability switches. The recurrence rule of the quantum scars in this complex solf-wall billiard differs from that of the hard-wall nonrelativistic quantum billiard, such as being equally spaced in energy instead of being equally spaced in the square root of energy. These results implement the previous knowledge and may be used for understanding the measurements of density of states and transport properties in two-dimensional electron systems with random long-range impurities.
The semiclassical treatment of the two-dimensional harmonic oscillator provides an instructive example of the relation between classical motion and the quantum mechanical energy spectrum. We extend previous work on the anisotropic oscillator with incommensurate frequencies and the isotropic oscillator to the case with commensurate frequencies for which the Lissajous curves appear as classical periodic orbits. Because of the three different scenarios depending on the ratio of its frequencies, the two-dimensional harmonic oscillator offers a unique way to explicitly analyze the role of symmetries in classical and quantum mechanics.